1. Field of the Invention
This invention relates to an injection screw in a heating cylinder of an injection apparatus used in molding of synthetic resin.
2. Background Art
In an injection screw, a rear end to a front end of an effective length of the screw is divided in order into a feed zone, a compression zone and a metering zone, and a screw (a flight) with a same outside diameter is formed successively across each zone thereof. Moreover, although as a screw pitch, normally, a pitch (P)/an outside diameter (D)=1 is used, with the objective of improving the plastificating efficiency or the like a root diameter of the screw of the metering zone is formed in a larger diameter than a diameter of the feed zone, and a root diameter of the screw of the compression zone therebetween is enlarged in order from the feed zone toward the metering zone, thereby to be tapered.
Each zone thereof has the functions as follows: melting of a pelletized molding material, that is, a resin in a heating cylinder is caused by both a band heater mounted on an outer peripheral of the heating cylinder and heat generated by shearing of the resin caused by rotation of the screw, and in the feed zone, the pelletized molding material fell in a rotating screw channel from a hopper is fed forwardly and is preheated simultaneously, and the resin is transferred to the compression zone while being melting by both the exterior heating and the heat generated by shearing of the resin: in the compression zone, the semi-molten resin is melted and kneaded by the exterior heating and shearing action while being compressed by the tapered channel to be sent to the metering zone: and in the metering zone, the molten resin is kneaded further completely and is made uniform to flow out from the screw head into a head section of the heating cylinder with uniform flowing speed.
Although it has been said that the resin plastificated by such an injection screw is made uniform by the exterior heating on the occasion of passing through the metering zone of which the screw channel depth is set most shallowly, resin can not be necessarily made uniform depending upon the set length of the metering zone, whereby variations in the resin temperature at injecting is occurred even in the standard screw as shown in FIG. 3.
Although it is thought to set the metering zone long, as a means for solving this problem, a time period that the plastificated resin is heated by the exterior heating becomes long due to the elongated metering zone, so that resin is heated more than required, and moreover, flowing resistance also is increased, resulting in the cause inducing generation of the excessive heat by shearing in the compression zone, thereby not only there being the risk that resin Is heated excessively, so that burning or the like are caused and resin is deteriorated, but also the new problems that the adverse effect is exerted on supplying of the pelletized molding material at the hopper, or the like being caused